1. Field of the Invention
The present invention relates to a method of manufacturing an optical recording medium and particularly to a method of manufacturing an optical recording medium having at least two minute concave and convex surfaces or more.
2. Background of the Invention
An optical recording medium used for recording and/or reproducing information by irradiating rays of light includes a read-only memory (ROM) type, a write-once type, and a rewritable type. In the ROM type optical recording medium such as a compact disk, for example, a minute concave and convex surface such as a phase pit where a data information, a tracking servo signal and so on are recorded, a pregroove or the like is formed in its information recording layer. In the write-once type or rewritable optical recording medium such as a magneto-optical recording medium, a phase-change recording medium, a minute concave and convex surface such as a pregroove or the like is formed.
FIG. 1 is a schematic, cross-sectional view of an optical disk, e.g., a compact disk (CD).
When this optical recording medium is formed, a transparent substrate 1 made of polycarbonate or the like and having a thickness of 1.2 mm is formed by injection molding, and at the same time a minute concave and convex surface 2 such as a pregroove is formed. A reflective film 4 formed of an AL evaporated film or the like is formed on the minute concave and convex surface 2 so as to have a thickness of several tens of nm, thereby an information recording layer 5 formed of the minute concave and convex potion 2 and the reflective film 4 being formed. Further, a protective film 6 made of an ultraviolet curing resin, for example, and having a thickness of several .mu.m is laminated on the information recording layer 5. Thus, the optical recording medium is formed.
An optical recording medium having a plurality of laminated information recording layers has been proposed in order to increase a recording information amount.
FIG. 2 is a schematic, cross-sectional view of an optical recording medium having a dilayer structure.
This optical recording medium is arranged such that first and second information recording layers 11, 12 are laminated on a transparent substrate 10 with a transparent intermediate film 33 being disposed therebetween.
The first information recording layer 11 consists of a first minute concave and convex surface 21 formed at the same time when the substrate 10 is formed by injection molding of resin such as polycarbonate or the like, and a semitransparent film 13 made of, for example, SiN and deposited thereon. The second information recording layer 12 is formed of a second minute concave and convex surface 22 laminated on the first information recording layer 11 by photopolymerization (2P) method, and a reflective film 14 formed of an Al evaporated film or the like and deposited thereon. A protective film 16 made of an ultraviolet curing resin, for example, is formed on the second information recording layer 12.
As shown in FIGS. 1 and 2, reading light L is irradiated on the optical recording medium from its substrate side thereof, thereby information being read out from the information recording layers thereof. In this case, as shown in FIG. 2, when information is read out from the optical recording medium having the first and second information recording layers 11, 12, respective rays of light L focused on the first and second information recording layers 11, 12 are irradiated on the first and second information recording layers 11, 12 from the same side, thereby the information being read out from the first and second information recording layers 11, 12.
However, each of the optical recording medium having a single information recording layer shown in FIG. 1 and the optical recording medium having two information recording layers shown in FIG. 2 has an asymmetrical structure in its thickness direction in view of its material. Therefore, immediately after the optical recording medium is manufactured, shrinkage of the ultraviolet curing resin forming the protective film 16 tends to warp the substrates 1 and 10 and to keep them in their warped states. Secular changes thereof mainly brings their deformation such as warps resulting from absorption or removal of vapor in the air. Therefore, it leads to an unsatisfactory operation of reading a signal by an optical pickup.
When the optical recording medium having a dilayer structure shown in FIG. 2 is formed, the first and second minute concave and convex surfaces 21, 22 forming the first and second information recording layers 11, 12 must be independently and respectively formed by injection molding or the 2P method. Therefore, it leads to low productivity.